support a whitelist of owners for validate

[jjmschofield]

To help people support the schema at https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners

Where in:

"Invalid syntax includes inline comments and user or team names that do not exist on GitHub."

In a config (somehwere, somehow) create a list of allowed owners and make the validate command respect it.

{
  ...
  "validate":{
    ...
    "owners": {
      "allowed": [
        "@bob"
      ]
    }
  }
}